HomeostasisSpec 2.81
In short
Homeostasis is the maintenance of a constant internal environment. Examples are the control of body water content and body temperature, which change when we sweat, drink, exercise or move between hot and cold places. It matters because enzymes and cells work best in a narrow range of conditions, so reactions slow down or stop if conditions move too far.
- Homeostasis
- The maintenance of a constant internal environment.
Conditions inside the body change when we eat, exercise, or move between hot and cold places. The body responds to keep the conditions inside the body constant.
Two examples of homeostasis| Condition | Example of a change |
|---|
| Body water content | Sweating, or drinking a lot |
|---|
| Body temperature | Hot or cold surroundings, or exercise |
|---|
This matters because enzymes and cells work best in a narrow range of conditions. If conditions move too far from normal, reactions in cells slow down or stop.
Exam tip:Constant does not mean the level never changes. It means the body brings it back to a set level, so say 'kept within narrow limits'.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026
Frequently asked questions
What is the difference between a receptor and an effector?
A receptor is a cell or organ that detects a stimulus, while an effector is a muscle or gland that carries out the response. For example, temperature receptors in the skin detect a hot object, and a muscle in the arm contracts to pull the hand away. The brain and receptors are never effectors.
Why is homeostasis important?
Homeostasis is important because enzymes and cells work best in a narrow range of conditions. If body temperature or body water content moves too far from normal, reactions in cells slow down or stop. Homeostasis keeps the internal environment constant by bringing conditions back to a set level, keeping them within narrow limits.
How does auxin make a shoot bend towards light?
Auxin makes a shoot bend towards light by causing unequal growth. It is made in the shoot tip, and when light shines from one side it moves to the shaded side. There it makes the cells elongate more, so the shaded side grows faster than the lit side and the shoot bends towards the light.
All 4 questions on Coordination, homeostasis and plant responses
